Our recent work with photoreceptors has indicated that cyclic AMP mediates at least some of the effects of light on these cells. In this system there is a light-regulated, rhodopsin-linked phosphodiesterase which has been eluted from the disc membrane and purified to homogeneity. This system provides unique opportunities for elucidation of the mechanism by which intrinsic (rhodopsin) and extrinsic (phosphodiesterase) membrane proteins interact. This research represents the first demonstration of coupling of the photoisomerization of rhodopsin to the enzymatic activity of a specific disc membrane enzyme component. In addition, our finding that the interaction of light and ATP (which activate disc phosphodiesterase) can be mimicked by a variety of macromolecular polyanions provides the possibility of formulating a mechanistic hypothesis concerning the role of charge displacement in the interaction of membrane proteins. We are attempting to characterize all of the elements in the activation sequence and the anchor protein to which phosphodiesterase is bound.